Communication system networks are known to comprise a plurality of communication systems and a central controller. Each of the communication systems comprises a plurality of communication units, a limited number of communication resources that are transceived via a limited number of repeaters, and a communication resource allocator that allocates the communication resources among the plurality of communication units. The communication units, which may be mobile radios, portable radios, and/or portable radio/telephones, are typically arranged in to communication groups and are generally located throughout the communication system network. Of the communication resources, which may be TDM slot locations, frequency carriers, pair of frequency carriers, or any other RF transmission means, one is selected to function as a control channel. The control channel transceives operational information between the communication units and the central controller via the communication resource allocators.
As is known, a communication unit may establish a communication with other communication units within the communication system network by transmitting an inbound signalling word (ISW) to the central controller via the communication resource allocator of the communication system that the communication unit is located in. Upon receiving the request, the central controller processes it and, if the request is valid, the central controller allocates a communication resource in each of the communication systems that have a communication unit of the targeted talk group in it. The communication systems, which may also be communication sites, are coupled to and controlled by the central controller. If a communication system, or site, loses contact with the central controller, the communication system, or site, operates as an independent communication system but only for communication units that are located within its coverage area.
As is also known, a communication system network may interface with a telephone system such that communication units within the network can access telephone lines. These telephone resources are allocated by the central controller to a communication unit in a similar manner as the communication resources are allocated. When a communication unit has access to a telephone resource and a communication resource in the communication system that it is located in, it may communicate to a land-line user. A difficulty arises when the communication unit roams from one communication system to another. Under such conditions, the communication unit loses the communication resource of the system that it was in and, subsequently, the central controller would reclaim the telephone resource even if the communication unit was not finished with its communication. This obviously presents a problem to the user of the communication unit while he is communicating via a telephone resource. Therefore, a need exists for a method that will allow a communication unit to maintain allocation of a telephone resource when it roams into another communication system.